A series of monoprotonated aliphatic diamines has been examined, which crystallize in three general motifs: salt-bridged, cyclic, or clustered. The monoprotonated triflic acid salt of Me2N (CH2)(4)NMe2 forms a proton-bridged cyclic cation. The internal N -N distance is 2.66 angstrom, with the bridging proton in the middle, having an NHN angle >= 172 degrees. The triflate oxygens lie more than 4 A away from the midpoint between the nitrogen atoms, indicating that a salt bridge does not form. The average NH distance in a solid sample was determined by measuring the N-15-H dipolar coupling in the triflic acid salt of the completely deuterated diamine (CD3)(2)N(CD2) N-4(CD3)(2). The value of the dipolar coupling constant, 5250 +/- 90 Hz, corresponds to an average NH distance of 1.32 angstrom, nearly half-the NN distance. That result agrees with DFT calculations, which give a double-well potential minimum for proton transit between the two amino groups, having a zero-point vibrational level close to the barrier top. Theory predicts that the maximum value of the zero point vibrational wave function is almost coincident with a local potential energy maximum, consistent with the experimental findings.